A while back I installed a rebuilt and enlarged engine along with a dual carburetor setup in my 1971 VW Bus. But, for some reason the newly installed dual carburetors were not opening fully and the Bus wouldn't go faster than 50 mph. Initially, I suspected the long throttle cable had become kinked inside the tube and was hanging up, so I replaced it. That didn't help.

My next suspicion was that the linkage wasn't allowing enough travel to fully open the carbs. But, the linkage was adjusted as far as possible and the carbs were still not opening all the way. Suspect number three was the throttle pedal. It wasn't going all the way to the floor! I lubed the hinge pin but it still had limited motion.

Finally, I shined a flashlight under the pedal (a stamped steel unit with a short apron around the sides). Hiding right at the hinge, under the pedal, was a small chunk of gravel. I removed the debris and my top speed immediately increased by about 30 mph!

Stuart in Washington

All wheel drive (AWD) systems send power to a primary set of wheels (front or rear depending on the vehicle). When one or both of those primary wheels slips (spins faster than the others), then the computer directs some power to the secondary set of wheels. This is most often done using an electromagnetic clutch device that the computer engages to link the transmission to the axle(s) that drive the secondary set of wheels.

Instead of an electromagnet, some car companies (Audi, Volkswagen and Volvo as well as select models by Buick, Cadillac and others) use hydraulic pressure to engage/disengage their clutch device. This is commonly referred to as Haldex AWD because Haldex was the original manufacturer. (Borg-Warner bought the technology from Haldex and now makes most or all "Haldex" AWD systems.)

Haldex AWD System. The two shafts are connected via the wet clutch pack.

Haldex systems typically use an electric pump (AWD Coupling Oil Pump) to keep hydraulic fluid constantly pressurized so it is quickly able to engage a wet clutch pack at the computer's command. Relatively frequent maintenance is usually recommended by the vehicle owners manual to ensure that the AWD coupling fluid is clean and the system is correctly pressurized.

Any contaminants or failing components can lead to a similar cascade of problems. Dirty fluid can clog the filter/strainer which prevents the pump from providing adequate pressure, or a pump that has an intermittently failing electric motor may not be able to provide enough pressure. If the fluid pressure is incorrect for any reason, then there could be excessive slippage in the clutch pack, and that might generate too much clutch friction-material grit floating around in the fluid. That grit clogs the filter/strainer which prevents the pump from providing adequate pressure...

The owners manual will often recommend both mileage and time intervals for maintenance (~3years or ~40,000 miles/64,000 km is typical). It is important to not ignore the suggested time interval especially if the vehicle does not travel great distances but spends a lot of time sitting in traffic or slowly navigating off-road trails. The electric pump is likely rapidly cycling on and off to maintain fluid pressure whenever the transmission is in gear. Pump wear and potential fluid contamination can occur even if the vehicle has low mileage or is rarely driven in slippery conditions.

Maintenance usually includes draining the AWD coupling fluid and removing the AWD coupling oil pump and cleaning or replacing the filter/strainer at the pump's inlet. Some systems have an additional filter cartridge to replace. The oil pump usually needs new o-rings installed to best prevent leaks.

New fluid is squirted into a fill hole using a suction gun, fluid transfer pump or similar tool. Care must be taken to use the drain and fill holes for the Haldex unit rather than the drain and fill holes for the nearby differential. To fill the fluid to the correct level (typically the rim of the fill hole), the vehicle usually must be level and the Haldex unit relatively warm. The fluid might need to be topped off after the pump runs and refills itself with fluid.

Haldex Pump, Fluid & Maintenance Kit

Look under "Drivetrain" in the RockAuto.com catalog to see new AWD coupling oil filters, fluid and pumps for your specific vehicle. With the part choices may be kits that conveniently include new filter, strainer, o-rings, oil, drain/fill plugs and other necessary parts. Some kits include a replacement pump. Be sure to read the vehicle's owners manual/repair manual for specific instructions on how to perform maintenance on a Haldex type AWD system.

Tom Taylor,
RockAuto.com

To read more of Tom's articles, click this link and choose from story titles on the Newsletter Archives page.